The testing of particle filter systems for leaks (e.g. openings in the filter, gaps or imperfections in the sealing of the air path) generally involves reducing or eliminating the contribution of particles which penetrate through the filter medium itself. The reduced contribution is typically accomplished by using test particles which are known to be removed efficiently by the filter medium.
A common method of quantitative fit-testing involves taking particle concentration measurements, both inside of a respirator mask and just outside of the mask. This is accomplished by using a vacuum pump to draw an aerosol sample from the atmosphere just outside the mask, and then drawing another aerosol sample from within the mask, either by using a sampling adapter or a test mask with a face piece modified to receive a sampling tube. The two samples are provided, alternately, to a particle measuring device such as a condensation particle counter that generates particle counts indicating the respective concentrations of the tested aerosol samples.
The two counts or measurements are compared by providing a ratio of the count outside the mask to the count inside the mask, known as the “fit factor”. A higher fit factor indicates a filtration that more effectively seals against leakage.
The validity of this test is based largely on an assumption that the count or concentration inside the mask is due to leakage rather than penetration through the filter, i.e. an assumption that the filter is nearly 100 percent efficient. The National Institute for Occupational Safety and Health (NIOSH) has classified particulate air-purifying respirators according to 42 CFR Part 84. Three major classes exist within this standard: class 95, 99 and 100. One of the class 95 respirators, known as “N95” filtering facepieces, consist of a mask composed entirely of the filter medium, without a supporting elastomeric mask. The N95 respirator is greater than 95 percent efficient at the most penetrating particle size. Recent regulatory changes have resulted in an upsurge of N95 respirator production and usage, and government regulations continue to require fit-testing.
Typically, filter materials have a minimum efficiency (corresponding to a maximum particle penetration) at a point along a particle size spectrum. Efficiency rises (reflecting reduced penetration) in both directions from the minimum efficiency point. Typically the minimum efficiency point occurs within a particle size range of 0.1- to 0.3-micrometers (μm).
It is known that when a class 95 respirator is exposed to a polydisperse aerosol, a relatively large number of particles at and near the minimum efficiency point pass through the filter and are detected inside the respirator mask. As used in this application, the term “aerosol” refers to a suspension of elements (e.g. solid particles or droplets) in a gaseous medium. Atmospheric or ambient air in an unconditioned state is an example of an aerosol, with air as the gaseous medium supporting typically 3,000-10,000 elements per cubic cm. Ambient air is also an example of a “polydisperse aerosol”, because the particles or other elements vary widely in size. By contrast, a “monodisperse aerosol” is comprised of particles at or near a particular diameter. Under certain conditions, the number of particles entering the mask through the filter can be substantially larger than the number entering the mask due to face-seal leakage or other leaks, resulting in a distortion of the calculated fit factor that erroneously indicates a poor fit.
There are several approaches to fit testing class 95 respirators intended to remedy the problems associated with polydisperse aerosol testing. One approach requires that the mask be fitted with a class 99 or class 100 filter for fit test purposes. Such an approach is obviously incompatible with N95 respirators, where there is no supporting elastomeric mask into which the superior filter can be substituted.
Another approach involves generating a suitable monodisperse aerosol, e.g. with all particles at or about 2.5 micrometers in diameter. Dust/mist filters, N95 filters, and other low efficiency filters are considerably more efficient with respect to particles at or near 2.5 microns in diameter. Results based on this type of testing, however, are reliable only if testing occurs within a controlled or conditioned atmosphere including only the monodisperse aerosol. Maintaining this conditioned atmosphere is expensive, requiring an aerosol generator to produce the monodisperse aerosol and a chamber or other enclosure surrounding the person wearing the respirator under test. The enclosure limits the individual's ability to perform certain exercises or movements during fit-testing. This technique is described in an article entitled “Validation of a Quantitative Fit-Test for Dust/Fume/Mist Respirators: Part I”, Iverson et al; Applied Occupational Environmental Hygiene, March 1992, pp. 161-167.
Still another approach is based on the discovery that for DM and DFM respirators, the relationship between filter penetration and leakage depends upon the face velocity (flow rate). The approach is described in an article entitled “Fit-Testing for Filtering Face Pieces: Search for a Low-Cost, Quantitative Method”, Myojo et al; American Industrial Hygiene Association Journal, 55 (9), 1994, pp. 797-805. Tests were conducted on mannequins and human subjects, both breath-holding and normal breathing. The technique, however, is limited primarily to aerosols in the submicrometer size range. Also, the reliability of tests on human subjects breathing normally depends on the ability to predict and monitor the subject's inhalation rate.
Another known fit-test involves using an optical particle counter in combination with lower efficiency filters, such as dust/mist and N95. The complete polydisperse aerosol is sampled. Due to the limited capacity of the optical particle counter, i.e. its ability to detect only relatively large particles (more than 0.5 microns in diameter), the tendency of penetrating particles to bias leakage test results is reduced. However, the relatively small number of large particles occurring naturally in ambient conditions limits the utility of this approach, because the number of sensed particles is not sufficient to afford statistical accuracy.
U.S. Pat. No. 6,125,845 to Halvorsen et al., assigned the assignee of the instant application and the disclosure of which is hereby incorporated by reference in its entirety except for express definitions contained therein, discloses a system and process for respirator fit-testing that utilizes ambient airborne particles. Halvorsen discloses a system that acquires aerosol samples from inside and outside of the respirator mask, respectively, routing the samples through a radial differential mobility analyzer to modify the samples so that only particle sizes of a predetermined element characteristic (e.g., size) are to be analyzed, and routing the modified samples through a condensation particle counter to determine the concentrations of suspended elements in the respective modified samples. The quotient of the outside to the inside concentration values yields the fit factor.
An advantage of the device disclosed by Halvorsen is that there is no need to generate an aerosol for testing purposes. A problem can arise, however, when the unconditioned ambient atmosphere is of a low particle concentration. A paucity of particles in the aerosol samples can lead to low resolution of the fit factor and/or increase the testing time required to obtain particle counts adequate for a reasonable measure of the fit factor.
An apparatus and method that overcomes problems associated with sparse ambient particle concentrations would be welcome.